1. Field of the Invention
The present invention relates to a Langasite single crystal ingot having a very small variation in the contents of the components, lanthanum oxide (hereinbelow, La2O3), gallium oxide (hereinbelow, Ga2O3), and silicon oxide (hereinbelow, SiO2), through the entire straight part, excluding the shoulder part that is formed during the early growth by pulling up a seed crystal, and thereby has a superior homogeneity in the contents of the components. The present invention further relates to a method for manufacturing a substrate of a piezoelectric device suitable for a SAW filter and a piezoelectric device substrate, and a surface acoustic wave device using the same.
The present application is based on patent applications filed in Japan (Japanese Patent Application No. Hei 11-377335; Japanese Patent Application No. 2000-001426; Japanese Patent Application No. 2000-165114; and Japanese Patent Application No. 2000-165119), the contents of which are incorporated herein by reference.
2. Description of the Related Art
Recently, single crystals of Langasite represented by the formula La3Ga5SiO14 (Langasite) and comprising in terms of weight (mass) ratios:
La2O3: 48.04%,
Ga2O3: 46.06%,
SiO2: 5.90%,
and having such characteristics as:
    (a) the rate of change due to temperature of the surface acoustic wave propagation velocity is low, and the rate of change due to temperature of the frequency is low; and    (b) the electromechanical coupling coefficient that represents the dimensions of the piezoelectric characteristics (the coefficient representing the mutual conversion rate between the electrical energy and the mechanical energy) is large, has become the focus of attention as a substrate material for piezoelectric devices such as surface acoustic wave (SAW) filters, as is disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 10-126209, and have become the object of research (for example, H. Takeda, K. Shimumura, V. I. Chani, T. Fukuda, “Effect of starting melt composition on crystal growth of La3Ga5SiO14”, J. Crystal Growth 197 (1999) 204). Specifically, a Langasite single crystal has temperature characteristics equivalent to those of quartz crystal, and furthermore, it has an electromechanical coupling coefficient about 3 times that of quartz crystal, and thus can be used for broadening the bandwidth of SAW filters that have many uses, such as in cell phones, and make them more compact.
It is known that when applying the above as a substrate material or the like, the straight part of the Langasite single crystal ingot, which is the part remaining after eliminating the ingot shoulder formed during the initial growth by pulling up the seed crystal, is sliced, and supplied for actual use in the form of a wafer of a predetermined thickness. Then the Langasite single crystal ingot is manufactured by the seed crystal of the Langasite being fastened to the distal end of the pull shaft, and when this seed crystal is brought into contact in a crucible with the surface of the Langasite melt having a predetermined composition, and ingot growth is carried out by pulling up the pull shaft while it is being rotated.
Conventionally, to grow this Langasite single crystal, the single crystal is grown by melting raw material pellets based on the composition of the stoichiometric composition ratio.
The progress of single crystal growth by pulling up technology in recent years has been remarkable, and the demands for lower energy consumption, lower power consumption, and lower cost have become strong. Accompanying this, there has been a trend to increase the diameter of the Langasite single crystal ingot even further. However, in the conventional Langasite single crystal ingot described above, variation in the contents of La2O3, Ga2O3, and SiO2, which are the components, over the entire ingot occurs easily, and in particular, the variation of the content of the Ga2O3 is severe. Especially, when the diameter of the ingots becomes large, the variation becomes remarkable. Therefore, in the current situation, the growth of a single crystal ingot having a stable composition over the entire ingot has been difficult.
In addition, when growing a Langasite single crystal with the composition conventionally used having the stoichiometric composition or the like, the fluctuation of the growth rate that accompanies a discontinuous saturation degree is associated with fluctuations in the composition, and there is the disadvantage that growth striations occur. When growth striations are present in the surface of a wafer that is cut out from this crystal, there are variations in the mechanical strength, and when it undergoes a polishing process, surface irregularities are produced, and there is the problem that this has a deleterious influence on the properties of a SAW filter. In addition, the convection velocity of the melt is influenced in the direction of the diameter, and thus the composition differs at each point on the surface, and as a result, the surface acoustic wave velocity in the wafer surface also becomes irregular.
Furthermore, when a Langasite single crystal is grown using the conventional stoichiometric composition or the like, differences in the composition of the substrate surface and the top section and the bottom section of the growth crystal occur that are caused by differences due to the congruent melt composition (congruent composition). As a result, secondary phases and defects easily occur in the crystal, the crystal cracks easily, and in the case that the differences are large, entrapped inclusions can be seen. In addition, in order to suppress the generation of secondary phases, the growth rate must be set at an extremely low value, and there is the problem that the production efficiency deteriorates. Furthermore, because Ga has the property of being highly volatile among the atoms that form the Langasite single crystal, depending on the composition, there is the disadvantage that a stable growth by pulling up is difficult due to this high volatility.
These differences in the crystal composition have a significant influence on the properties of a SAW filter, and in particular the propagation velocity of an surface acoustic wave is dependent on the density of the material, and thus as a result of the fluctuations in the composition from the top section to the bottom section of the crystal, the density of the crystal fluctuates, and thereby the propagation velocity of the substrates that are cut out from the top section to the bottom section of the crystal will fluctuate. The center frequency, which is an important property of the SAW filter, is determined by the propagation velocity and the dimensions of the interdigital transducer of the element, and thus a SAW filter manufactured by the same interdigital transducer design using such a substrate will have a large propagation velocity variation and a large center frequency variation, and cannot be practically used.
In growth using a composition that deviates largely from the congruent melt composition, in the growth interface, the effective distribution coefficient of each of the components of the melt are influenced by the convection velocity of the melt, and thus the composition will also differ at each point of the interface. As a result, the surface acoustic wave velocity (propagation velocity) will become dispersed. Therefore, in the conventional technology that uses raw material pellets based on the composition of the stoichiometric composition ratio, obtaining a uniform Langasite crystal has been difficult.